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3-Diazopyrroles—IV. Structure determination using 13C NMR spectroscopy
Specmchimica Am, Vol. 46A. No. 6. pp. 995-997, Printed in Great Britain
3-Diazopyrroles-
05~8539/90 f3.00 + 0.00 @ 1990 Pergamon Press plc
1990
IV. Structure determination using 13CNMR spectroscopy
GIROLAMO CIRRINCIONE, ANNA MARIA ALMERICO and ENRICO AIELLO Istituto Farmacochimico,
Universita di Palermo, Italy
GAETANODATTOLO Istituto Chimico Farmaceutico e Tossicologico, Universita di Milano, Italy
and R. ALAN JONES* School of Chemical Sciences, University of East Anglia, Norwich NR4 7TJ, U.K (Received
16 June 1989; accepted 23 June 1989
Abstract-On the basis of the 13CNMR chemical shifts, it is proposed that, although b is the major canonical structure, structure c, in which a negative charge resides at C-3, provides an important contribution to the resonance stabilization of the 3-diazopyrroles, l-4.
INTRODUCTION IN THE
course of our research on polycondensed nitrogen heterocycles with potential biological activity, the 3-diazopyrroles play an important role as key intermediates [l]. Although some spectroscopic (mainly i.r.) data are available [2], the structure of this class of compounds has not been exhaustively investigated. Additionally, there is only limited information concerning ‘H and 13CNMR spectra of simple 3H-pyrroles [3] and no data has been reported for 2-azafulvenes or for 3H-pyrroles with an exocyclic C=X function at the 3-position.
EXPERIMENTAL
The diazopyrroles, 1-4, were synthesized according to procedures in the literature [l]. The ‘H and 13C NMR FT spectra were measured for lo-40 mg samples in DMSO-d6 (0.5 ml) at room temperature using a Jeol JNM-GX400 spectrometer. Chemical shifts are recorded relative to the internal TMS standard. The 2-D experiment was conducted using the VCHSHF pulse sequence. 256 tl values were recorded using 2048 complex points and 128 scans for each value. The fi (‘H) spectral width was 3453 Hz and the f2 (“C) spectral width was 9746 Hz. PI3 was set to 1.8 ms.
RESULTS AND DISCUSSION
We have now measured the ‘H and 13C NMR spectra of the 3-diazopyrroles l-4 as DMSO-d, solutions at 400 and 100 MHz, respectively. The ‘H NMR spectra do not provide any significant information on the structure of these compounds other than the absence of signals due to iminic ring protons (Table 1). The addition of TFA to the solutions regenerated the diazonium species which, in the case of 1, provided a ‘H NMR spectrum identical to that for the corresponding lH-pyrrole-3-diazonium chloride measured in DMSO-d,: 7.45 (NH); 7.76 (4-CH); 8.135, 8.08 and 7.52 (phenyl). The “C NMR spectral data for the 3-diazopyrroles l-4 are recorded in Table 2. The signal assignments were made by a 2-D ‘H-13C signal correlation experiment on compound 1. * Author to whom correspondence
should be addressed. 995
GIROLAMO CIRRINCIONE et al.
+,N’
-
a
b
C
d
1. R’ = Ph, R* = H;
2.
R’ = Ph, R2 = CN
3.
4.
R’ = Me, R2 = C02Et
I? = Ph, R2 = C02Et
Formulae l-4.
Table 1. ‘H NMR chemical shifts (6) for 3-diazopyrroles and for the protonated species
1 2 3 4 l+TFA
NH
4-CH
o
7.45
7.26 7.76
7.94 8.12 7.90 7.89 8.13
Phenyl ring m P 7.83 7.91 7.53 7.54 8.08
Other substituents
74217.48 7.56 7.41 7.41 7.52
1.23, 4.23 (Et) 1.24,4.24 (Et); 2.09 (Me)
It would be expected that b would be the major dipolar canonical structure contributing to the resonance hybrid of the diazopyrroles and this expectation is supported by the symmetry of the 13CNMR spectrum in which the positions of the C-2 and C-5 signals between 142.7 and 150.6ppm, and 157.2 and 159.0ppm, respectively, are similar and differ significantly from those for the C-3 and C-4 atoms. The C-3 signal lies in the range 76.5-91.65 ppm, i.e. considerably upfield of the signal for the corresponding sp’ hybridized carbon atom of a lH-pyrrole [4], but downfield of the C-3 signal of 3,3-disubstituted 3H-pyrroles [3]. These shifts suggest a substantial contribution of the canonical structure c having a negative charge at C-3. It is noteworthy that the C-3 signals have exceptionally small intensities, compared with the signals for the other quaternary carbon atoms. The C-4 signals were observed in the range 88.2-110.6ppm, depending upon the nature of the substituent R*, and are consistent with a contribution to the resonance hybrid of structure d. The general conclusions regarding the electronic structure of the diazopyrroles and, in particular, the important contribution of structure c to the resonance hybrid contrast with the non-polar structure, which is commonly used to represent this class of compounds, but is in broad agreement with the dipolar structure proposed for diazocyclopentadiene [5]. The 13CNMR spectra of the protonated species were compatible with the lH-pyrrole structure possessing a conjugated cationic group at the 3-position, e.g. the spectrum for 1 Table 2. “C NMR chemical shifts (6) for the ring atoms of 3-diazopyrroles
Assignment c-2 c-3 c-4 c-5
1
2
142.7 76.5 106.4 157.2
150.6 81.5 88.2 158.8
Compound 3 149.3 79.9 110.6 159.0
4 149.9 91.65 110.55 158.7
3-Diazopyrroles-IV
991
to which TFA had been added showed signals at 151.9 (C-2), 88.5 (C-3), 107.9 (C-4), and 132.9 (C-5) ppm. Acknowledgements-The authors are grateful to Dr DAVID WILLIAMSONfor his general help in the operation of the spectrometer and, in particular, for the 2-D studies.
REFERENCES [l] G. Dattolo, G. Cirrincione, A. M. Almerico, G. Presti and E. Aiello, Heterocycfes 20,829 (1983); 22,2269 (1984); G. Dattolo, G. Cirrincione, A. M. Almerico and E. Aiello, Heferocycfes 20,255 (1983). [2] A. Kreutzberger and P. A. Kalter, /. Phys. Chem. 65, 624 (l%l). [3] M. P. Sammes, in Chemistry of Heterocyclic Compounds (edited by R. A. Jones), Vol. 48, ch. 4. Wiley, New York (1990). [4] R. A. Jones and G. P. Bean, The Chemistry of Pyrroles. Academic Press, New York (1977). [5] R. 0. Duthler, H. G. Forster and J. D. Roberts, J. Am. Chem. Sot. 100, 4974 (1978).
3-Diazopyrroles-
05~8539/90 f3.00 + 0.00 @ 1990 Pergamon Press plc
1990
IV. Structure determination using 13CNMR spectroscopy
GIROLAMO CIRRINCIONE, ANNA MARIA ALMERICO and ENRICO AIELLO Istituto Farmacochimico,
Universita di Palermo, Italy
GAETANODATTOLO Istituto Chimico Farmaceutico e Tossicologico, Universita di Milano, Italy
and R. ALAN JONES* School of Chemical Sciences, University of East Anglia, Norwich NR4 7TJ, U.K (Received
16 June 1989; accepted 23 June 1989
Abstract-On the basis of the 13CNMR chemical shifts, it is proposed that, although b is the major canonical structure, structure c, in which a negative charge resides at C-3, provides an important contribution to the resonance stabilization of the 3-diazopyrroles, l-4.
INTRODUCTION IN THE
course of our research on polycondensed nitrogen heterocycles with potential biological activity, the 3-diazopyrroles play an important role as key intermediates [l]. Although some spectroscopic (mainly i.r.) data are available [2], the structure of this class of compounds has not been exhaustively investigated. Additionally, there is only limited information concerning ‘H and 13CNMR spectra of simple 3H-pyrroles [3] and no data has been reported for 2-azafulvenes or for 3H-pyrroles with an exocyclic C=X function at the 3-position.
EXPERIMENTAL
The diazopyrroles, 1-4, were synthesized according to procedures in the literature [l]. The ‘H and 13C NMR FT spectra were measured for lo-40 mg samples in DMSO-d6 (0.5 ml) at room temperature using a Jeol JNM-GX400 spectrometer. Chemical shifts are recorded relative to the internal TMS standard. The 2-D experiment was conducted using the VCHSHF pulse sequence. 256 tl values were recorded using 2048 complex points and 128 scans for each value. The fi (‘H) spectral width was 3453 Hz and the f2 (“C) spectral width was 9746 Hz. PI3 was set to 1.8 ms.
RESULTS AND DISCUSSION
We have now measured the ‘H and 13C NMR spectra of the 3-diazopyrroles l-4 as DMSO-d, solutions at 400 and 100 MHz, respectively. The ‘H NMR spectra do not provide any significant information on the structure of these compounds other than the absence of signals due to iminic ring protons (Table 1). The addition of TFA to the solutions regenerated the diazonium species which, in the case of 1, provided a ‘H NMR spectrum identical to that for the corresponding lH-pyrrole-3-diazonium chloride measured in DMSO-d,: 7.45 (NH); 7.76 (4-CH); 8.135, 8.08 and 7.52 (phenyl). The “C NMR spectral data for the 3-diazopyrroles l-4 are recorded in Table 2. The signal assignments were made by a 2-D ‘H-13C signal correlation experiment on compound 1. * Author to whom correspondence
should be addressed. 995
GIROLAMO CIRRINCIONE et al.
+,N’
-
a
b
C
d
1. R’ = Ph, R* = H;
2.
R’ = Ph, R2 = CN
3.
4.
R’ = Me, R2 = C02Et
I? = Ph, R2 = C02Et
Formulae l-4.
Table 1. ‘H NMR chemical shifts (6) for 3-diazopyrroles and for the protonated species
1 2 3 4 l+TFA
NH
4-CH
o
7.45
7.26 7.76
7.94 8.12 7.90 7.89 8.13
Phenyl ring m P 7.83 7.91 7.53 7.54 8.08
Other substituents
74217.48 7.56 7.41 7.41 7.52
1.23, 4.23 (Et) 1.24,4.24 (Et); 2.09 (Me)
It would be expected that b would be the major dipolar canonical structure contributing to the resonance hybrid of the diazopyrroles and this expectation is supported by the symmetry of the 13CNMR spectrum in which the positions of the C-2 and C-5 signals between 142.7 and 150.6ppm, and 157.2 and 159.0ppm, respectively, are similar and differ significantly from those for the C-3 and C-4 atoms. The C-3 signal lies in the range 76.5-91.65 ppm, i.e. considerably upfield of the signal for the corresponding sp’ hybridized carbon atom of a lH-pyrrole [4], but downfield of the C-3 signal of 3,3-disubstituted 3H-pyrroles [3]. These shifts suggest a substantial contribution of the canonical structure c having a negative charge at C-3. It is noteworthy that the C-3 signals have exceptionally small intensities, compared with the signals for the other quaternary carbon atoms. The C-4 signals were observed in the range 88.2-110.6ppm, depending upon the nature of the substituent R*, and are consistent with a contribution to the resonance hybrid of structure d. The general conclusions regarding the electronic structure of the diazopyrroles and, in particular, the important contribution of structure c to the resonance hybrid contrast with the non-polar structure, which is commonly used to represent this class of compounds, but is in broad agreement with the dipolar structure proposed for diazocyclopentadiene [5]. The 13CNMR spectra of the protonated species were compatible with the lH-pyrrole structure possessing a conjugated cationic group at the 3-position, e.g. the spectrum for 1 Table 2. “C NMR chemical shifts (6) for the ring atoms of 3-diazopyrroles
Assignment c-2 c-3 c-4 c-5
1
2
142.7 76.5 106.4 157.2
150.6 81.5 88.2 158.8
Compound 3 149.3 79.9 110.6 159.0
4 149.9 91.65 110.55 158.7
3-Diazopyrroles-IV
991
to which TFA had been added showed signals at 151.9 (C-2), 88.5 (C-3), 107.9 (C-4), and 132.9 (C-5) ppm. Acknowledgements-The authors are grateful to Dr DAVID WILLIAMSONfor his general help in the operation of the spectrometer and, in particular, for the 2-D studies.
REFERENCES [l] G. Dattolo, G. Cirrincione, A. M. Almerico, G. Presti and E. Aiello, Heterocycfes 20,829 (1983); 22,2269 (1984); G. Dattolo, G. Cirrincione, A. M. Almerico and E. Aiello, Heferocycfes 20,255 (1983). [2] A. Kreutzberger and P. A. Kalter, /. Phys. Chem. 65, 624 (l%l). [3] M. P. Sammes, in Chemistry of Heterocyclic Compounds (edited by R. A. Jones), Vol. 48, ch. 4. Wiley, New York (1990). [4] R. A. Jones and G. P. Bean, The Chemistry of Pyrroles. Academic Press, New York (1977). [5] R. 0. Duthler, H. G. Forster and J. D. Roberts, J. Am. Chem. Sot. 100, 4974 (1978).